Coke prevention in steam cracking



p N. c. PANIPHILIS ETAL 52 COKE PREVENTION IN STEAM CRACKING Filed Jan.2, 1968 FIGURE I N. c. PAMPHILIS J. A. KIVLEN Patent Attorney UnitedStates Patent 3,527,832 COKE PREVENTION IN STEAM CRACKING Nicholas C.Pamphilis, Mendham, and John A. Kivlen,

Sparta, NJ., assignors to Esso Research and Engineering Company, acorporation of Delaware Filed Jan. 2, 1968, Ser. No. 694,727 Int. Cl.C07c 3/30; C10g 9/16 US. Cl. 260-683 Claims ABSTRACT OF THE DISCLOSURE Atechnique and apparatus is described for lessening the tendency of steamcracker efiluents to coke in the process equipment downstream from thesteam cracker furnace. A special baffled collection manifold is used onthe furnace outlet tubes.

BACKGROUND OF THE INVENTION This invention relates to steam cracking ofhydrocarbons. More particularly, it relates to apparatus for minimizingcoking in steam cracking processes. This invention relates especially tocollection manifolds used with steam cracking furnaces which greatlylower the extent of coking in process equipment downstream from thefurnace.

It is known to react hydrocarbons and steam in a steam cracking furnace.One such process is described, e.g., in US. Pat. 2,893,941, issued July7, 1959, to Kohfeldt et al. In a typical such steam cracking reaction ahydrocarbon is fed with water or steam at considerable velocitiesthrough a high temperature reaction zone. A mixed stream of hydrocarbonand steam is preheated and passed through banks or groups of tubes in ahigh temperature furnace wherein the mixture is elevated to temperaturesranging on the order of from about 1300 F. to about 1800 F., dependingupon the specific feeds used and products desired. After the reactionhas progressed to the desired degree', the gaseous reaction products arewithdrawn from the furnace and rapidly quenched to avoid any furtherreaction, polymerization, or degradation of the desired products. Thetime in which the gases remain at cracking temperatures between thefurnace and the quenching zone is of critical importance, and,accordingly, the geometry and configuration of the conduit between thesetwo points is extremely important.

To insure that the gases from the various banks or groups of tubes inthe furnace are conveyed to the quenching zone in essentially the sametime, it is desirable to provide a collection manifold immediately atthe outlet of the furnace to combine all of the gaseous streams from thefurnace tubes into a single stream and then to convey the single streamto the quenching zone by means of a transfer line.

In a particularly preferred type of collection manifold, thecross-sectional area of the manifold progressively increases in-thedirection of gas flow so that as the manifold picks up gases fromsuccessive banks of furnace heating tubes, the gas velocity through themanifold remains essentially constant. This prevents the formation ofstagnant gas areas in the manifold which would permit undesirablereactions and coking in the manifold. Experience has shown, however,that even in the optimum designs of ap paratus severe coking isencountered in the downstream lines, e.g., the transfer line from thecollection manifold to downstream process equipment. Numerous attemptshave been made to prevent coking or to alleviate the degree of coking,e.g., by adding various chemicals or other decoking agents to theprocess. The addition of such agents is not entirely effective, and alsoit is sometimes undesirable to have the anti-coking agents present inthe products as contaminants.

3,527,832 Patented Sept. 8, 1970 SUMMARY OF THE INVENTION It has nowbeen found that by providing a bafile in the collection manifold inaccordance with the present invention the coking in the downstreamtransfer line can be greatly reduced. This invention contemplates theuse of a collection manifold for a steam cracking furnace having abaffle positioned to divide the streams of incoming gases from thevarious tubes of the steam cracking furnace into approximately equalparallel fractions and maintaining these fractions in separate chambersin the collection manifold. The fractions of gases are subsequentlyrecombined at the outlet of the collection manifold as they pass intothe transfer line for conveyance to downstream quenching facilities orother process equipment.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understoodby reference to the accompanying drawings of which:

FIG. 1 is a plan view of the collection manifold of this invention shownin cross-section with the baflle in place and;

FIG. 2 shows a view taken at cross section 2-2 of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the figures indetail, therein is shown a manifold body 10 having a number of inletopenings 12 to receive gaseous effluent from the tubes of a steamcracking furnace. A bafile plate 16 is held rigidly in place by acontinuous weld 20 along the edge opposite the manifold inlet openings12, and spacer rods 18 are attached at points along the baffle to holdit in rigid position within the manifold body. FIG. 1 also shows tubes22 which connect the manifold inlet openings 12 to the outlet tubes fromthe steam cracking furnace (not shown). Transfer line 24 is shown inposition at the outlet end of the manifold in position to convey gaseouseflluent from the manifold to downstream quenching facilities or otherprocess equipment (not shown).

FIG. 2 shows clearly the relationship of baflle 16 with manifold body10, dividing the latter into approximately equal sized chambers. Spacerrods 18 are also shown holding the baflle 16 in a stable position toavoid undue wear, vibration, or damage to the bafiie.

In the embodiment shown in the figures, the front edge of the baflie 16is positioned just behind the inlet openings 12 and is aligned so as todivide the openings approximately in half across their horizontal plane.While the baffle should be approximately perpendicular to the inletopenings, variations of 10 either way from the perpendicular have beenfound acceptable. In general, best results are achieved in reducingcoking of downstream equipment if the bafile is positioned to divide theincoming gases from the steam cracking furnace into approximately equalfractions. Preferably, neither fraction should contain more than about60% of the total gas flow, nor less than about 40%.

The following comparative tests and example illustrate the effectivenessof this invention in reducing the extent of coking in the transfer linedownstream from the collection manifold of a vertical steam crackingfurnace.

A conventional gas oil feed is preheated and mixed with about /2 lb.steam per lb. of oil and passed through the coils of a conventionalvertical steam cracking furnace. The coil outlet temperature of thegaseous product from the furnace is about 1600 F. The hot gaseousefiluent passes from the various banks of tubes through connecting tubesinto a steel collection manifold identical to that illustrated in FIG.1, except that it contains no baffle. The gases pass through themanifold and into a transfer line wherein they are quenched downstreamby injection of a stream of cold quench oil in an amount sufiicient toreduce the overall product temperature to about 500 F. The quenchedproduct is then conveyed to a distillation tower for separation of thecomponent into various desired products, including, e.g., ethylene,propylene, butadiene, and other olefins and diolefins.

Coking in the transfer line is soon evidenced by the pressure drop ofthe materials flowing through the line. The coke continues to build upuntil finally termination of the operation is required. The transferline is then opened and examination shows extensive coking and pluggingof the line both upstream and downstream from the quenching zone.

The collection manifiold is then opened and fitted with a steel bafile,welded into position as illustrated in the figures, dividing themanifold body into essentially equal sized chambers. The manifold isthen reinstalled, and steam cracking operations are again commenced inprecisely the same manner as previously described. Coking again isevidenced by a gradual build up in pressure drop; however, the rate atwhich the coke is deposited is much slower than in the previous run madewithout a baffle in the collection manifold. In fact, the run iscontinued for a period 80% longer than was previously possible with theunbaflled manifold.

Replicate runs are made, and the total operating time in each case is50% to 100% longer when the collection manifold is adapted with a bafflecompared to the base run made without a bafile.

It has been fairly conclusively established that the astonishingeffectiveness of applicants bafiled manifold in lessening the extent ofcoking in the transfer line results from its virtual elimination ofspiral flow of the elfluent gases. For example, it is noted that when abafile is not used, coke is laid down on the walls of the transfer linein a spiral pattern, whereas with the baffle in place, the coke laydownis generally uniform. Furthermore, mock-up tests in transparent scalemodels indicate, by visual examination, that the gases in the transferline follow a spiral pattern without the bafile, whereas no spiralpattern is formed when the bafiie is used.

It is not entirely certain how the bafiled collection manifold of thisinvention prevents spiral flow; however, it is believed that by dividingthe gas stream, dual spirals, which have opposite directions, areformed, and upon recombination, they eliminate each other. Surprisingly,a number of other mechanical devices have been found to be of little orno effect in preventing spiral flow. For example, orifice plates withsingle or multiple holes have been positioned at the manifold outletsand have been found completely ineffective in preventing spiral flow.Various configurations of straightening vanes have also been usedwithout success.

Spiral flow is a particularly acute problem in the most preferredconfigurations of collection manifolds, i.e., those having nonuniform,progressively increasing crosssections. Problems are, however, alsoencountered in cylindrical manifolds of uniform cross-section, and it iscontemplated that the unique baflie arrangement of applicants inventioncan be used in any conventional manifold configura tions.

The reason for the problems encountered in coking when the gaseousstreams take on a spiral flow in the transfer line are not entirelyclear. However, it is believed that the spiral flow causes heaviercomponents, especially from quench oil, which are more susceptible tocoking, to be thrown centrifugally to the walls of the transfer line toform slower moving, or stagnant, zones which are highly susceptible toadditional reaction, polymerization, or cracking. Whatever the cause, itis clear that coking is definitely more severe when the effluent gasesfrom the collection manifold take on a spiral flow pattern in thetransfer line to the downstream process equipment.

What is claimed is:

1. A method for lessening coking of efiluent gases in the transfer linedownstream from a collection manifold of a steam cracking furnace whichcomprises collecting the effluent gases from said furnace at atemperature ranging from about 1300 F. to about 1800 F., dividing theflow of said gases in said manifold into approximately equal parallelfractions, and recombining said fractions prior to quenching of thegaseous efiluent, whereby spiral flow of the recombined fractions ofgases is avoided.

2. Apparatus for the collection and distribution of gaseous efiluentfrom steam cracking furnaces to downstream processing equipment whichlessens coking of said effluent and which comprises:

a manifold body having a plurality of inlets for the entry of saideffluent and at least one outlet for the passage of said efiluentdownstream;

a bafile dividing said manifold body into approximately equal sizedchambers, said baflie positioned substantially perpendicular to saidinlets whereby said inlets and outlet are in direct and openuommunication with both of said chambers simultaneously;

conduit means for effluent fiow from said steam cracking furnace to saidinlets; and

transfer line means for connecting said outlet to said downstreamprocess equipment; whereby a fraction of said gaseous effluent from thesteam cracking furnace enters each of said chambers through said inletsand then leaves said chambers through said outlet and passes to saiddownstream process equipment through said transfer line means.

3. The apparatus of claim 2 wherein said bafile is a metallic baflieattached to said manifold body by means of a continuous weld along theedge of the baffle opposite said inlets.

4. The apparatus of claim 3 including spacer rods for maintaining saidbaffle in rigid alignment with said inlets and outlet.

5. The apparatus of claim 2 wherein said manifold continuously increasesin cross-sectional area in the downstream direction.

References Cited UNITED STATES PATENTS 1,774,29l 8/1930 Pelzer 2081321,962,502 6/1934 Grebe et al 260-683 2,201,965 5/1940 Cook 260683DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, JR., AssistantExaminer US. Cl. X.R.

